1. Field of the Invention
This invention relates to an improvement in a motocross tire for motorcycles.
2. Description of the Prior Art
In the specification, the term motocross shall be understood to mean motorcycle races in which drivers compete with each other to see who can turn round a given course on highly undulating irregular ground as many times as possible within a predetermined time or to see who can run a given number of rounds within a minimum time.
A motocross tire used for such motorcycle races is different from a conventional motorcycle tire used on a paved road. That is, the motocross tire is provided at its tread with a block pattern comprising polygonal rubber blocks suited for grasping the ground and projected from the tread surface in a regular manner for the purpose of travelling on mud or gravel.
In the motocross tire, the tread has heretofore been designed by taking the following two points into consideration.
In the first place, care is taken to a ratio of a space portion formed between adjacent blocks and grasping mud or gravel when the tire runs to a block portion. In general, it is preferable to make the above mentioned space portion for the overall tread small for the purpose of alleviating wear of the block. In this case, however, the important ground grasping property of the tire becomes degraded and hence the desired object could not be attained.
On the contrary, if the space portion is increased, the ground grasping property tends to increase upto a given limit. But, the blocks as a whole decrease in volume inducing premature wear of the tire. As a result, a bad influence is exerted upon the ground grasping property at the latter part of the motocross race.
Secondly, the motocross tire becomes rapidly worn when compared with the wear of the usual tire. It has heretofore been proposed to make rubber of the block considerably hard for the purpose of preventing degradation of the running ability of the motocross tire due to its wear when used. The use of such a measure, however, involves breakage of the block and degradation of the ground grasping ability of the tire.
It has also been proposed to provide a tread pattern comprising a group of blocks composed of a plurality of blocks having a square or rectangular outer surface and arranged side by side in parallel with the rotary axis of the tire and circumferentially spaced apart from each other by a regular distance.
The inventors' investigations and research have demonstrated the result that in the motocross tire mounted on a driving wheel (usually a rear wheel) which is important for the motocross race, a ratio of the driving force to a braking force, both being subjected to the tire when it runs, is about 70 to 30, that is, the driving force is far larger than the braking force. Also, the side surface of the ground contact surface subjected to the driving force when the tire runs, that is, that side surface of the block which precedes in the direction of rotation of the tire is often subjected to the driving force with the above mentioned ratio if compared with that side surface of the block which is opposed to the above mentioned side surface of the block and subjected to the braking force. If the motorcyle body is inclined to ground so as to turn its direction, that is, if the equatorial plane of the tire is inclined at an angle to a plane perpendicular to the horizontal surface (this inclined angle is called as a camber angle which sometimes becomes a large angle of about 40.degree. ), centrifugal force is subjected to the motorcycle body in the horizontal direction and a camber thrust is produced between the ground surface and the tire in a direction opposed to the centrifugal force. Among the blocks of the tread, that side surface of the block which is mainly subjected to the camber thrust is located at that side surface of the block which lies in the specified region.